Accumulation Tank vs. Domestic Hot Water Cylinder

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Accumulation tank vs. domestic hot water cylinder: what's the difference and why they're not the same thing

Almost every week at atria.sk we run into the same misunderstanding: a customer orders a "big tank for water" and means either an accumulation tank or a domestic hot water (DHW) cylinder - and while the two devices look similar on the outside (a cylindrical steel vessel, insulation, a few connections), they do completely different jobs. Mixing up these two components when designing a heating system is one of the most common reasons why, years into operation, someone complains about a shortage of hot water, a boiler that switches on and off too often, or, conversely, about an accumulation tank that's never properly used. In this article we'll explain the difference in practical terms, using real figures and typical operating situations, without trying to sell you anything - we don't currently have stock available in the accumulation tank category, so we can focus purely on making sure you understand the principle and can make the right decision, wherever you end up buying the device.

What an accumulation tank is and what it's for in the system

An accumulation tank (sometimes also called a "buffer tank") is an insulated steel vessel filled with heating water, which serves as a store of thermal energy for the heating circuit itself - i.e. for radiators, underfloor heating or air handling units. It doesn't hold drinking water, and water for washing or showering is never drawn from it directly. Its only job is to accumulate heat produced by the source (a boiler, heat pump, fireplace insert with an exchanger, solar collectors) and gradually hand it over to the heating system according to the household's current need.

The typical volume of an accumulation tank for an ordinary family house ranges from 500 to 1,500 litres; for solid-fuel boilers (wood, coal) 800 to 1,000 litres per fuel load burned is usually recommended, so the boiler can burn at full output and the surplus heat gets stored instead of escaping up the chimney during damped-down, inefficient burning. For heat pumps a smaller volume is chosen, usually 200 to 500 litres, since here it's more about hydraulically separating the circuits and reducing the number of compressor starts than genuine long-term heat storage.

An important property of an accumulation tank is so-called temperature stratification (layering) - warmer water naturally sits at the top, cooler at the bottom, and a correctly designed connection layout (several inlets and outlets at different heights on the tank) makes use of this layering so the heat source always draws the coldest water from the bottom and the heating circuit draws the hottest water from the top. Thanks to this, an accumulation tank can work across a wide temperature range, commonly from 35 °C (low-temperature underfloor heating) up to 80-85 °C (radiators fed by a solid-fuel boiler during intense burning).

What a DHW cylinder is and what it's for

A domestic hot water cylinder (commonly also called a "boiler" or "tank", although technically these terms in English can also mean an electric water heater) is a vessel in which drinking water intended directly for household consumption - showering, bathing, washing dishes, laundry - is heated and stored. Unlike an accumulation tank, a DHW cylinder is made of a material suitable for contact with drinking water (most often enamelled steel or stainless steel) and has to meet hygiene standards, including protection against the growth of Legionella bacteria.

It's exactly this Legionella issue that's the reason a DHW cylinder can't operate across the same wide temperature range as an accumulation tank. This bacterium multiplies best at temperatures of 25-45 °C, so it's essential for a DHW cylinder to regularly (at least once a week is recommended; many systems do this automatically once a day at night) reach a temperature of at least 60 °C throughout its whole volume, ideally briefly even 65-70 °C, to reliably kill off any bacteria. The normal operating temperature of a DHW cylinder is 55-60 °C, which is also the temperature at which the water is pleasant to mix with cold water when drawn.

The typical volume of a DHW cylinder for a family house is derived from the number of people and the household's habits: for 2-3 people, 120-150 litres is enough; for an ordinary family of four, 150-200 litres is recommended; for larger households with several bathrooms or a bathtub, 200-300 litres. For indirectly heated cylinders (with an exchanger connected to a boiler or heat pump), the exchanger's surface area also becomes an important parameter, determining how fast the cylinder can reheat after a large draw (for example after filling a bathtub).

Key differences at a glance

The following table summarises the most important differences between the two devices as described above:

How both devices fit into the heating system

The difference is best understood from a diagram of the heat flow from the source to the consumers. In a typical system with both an accumulation tank and a separate DHW cylinder, the heat's journey looks like this: the heat source (boiler, heat pump or a fireplace with an exchanger) heats water, which travels to the accumulation tank, from where it's distributed in two directions - to the heating circuit (radiators, underfloor heating), and to the DHW cylinder, where drinking water intended for household use is heated indirectly via an exchanger.

Notice an important detail in the diagram: the accumulation tank is a closed hub, from which branches lead to two completely different consumers - and the water circulating between the source, the tank and the heating circuit never mixes with the drinking water in the DHW cylinder. Heat transfer between them happens only through the wall of an exchanger (a coil or plate exchanger), similar to a classic "boiler with a cylinder", just with an accumulation tank added as an intermediate stage.

Why "combined" solutions sometimes exist

There are also devices on the market that outwardly look like a single vessel, but actually combine both functions inside one shell. The best-known principle is the so-called "tank-in-tank" - a smaller stainless-steel or enamelled vessel, used exclusively for drinking water, is inserted inside a larger accumulation tank and completely surrounded by the accumulation tank's heating water. The advantage is saving space (one vessel instead of two) and faster DHW heating thanks to the large heat exchange area (the whole surface of the inner vessel), the disadvantage is a higher purchase price and, if the inner vessel fails, more complicated servicing.

Another type of combination is an accumulation tank with a built-in coil exchanger ("with a coil") directly inside - here drinking water is heated on a flow-through basis as it passes through a coil submerged in the heating water, without its own large volume for storing hot water. This solution is more suitable for smaller draws (e.g. one bathroom, a flat); for a larger sudden draw (filling a bathtub, several people showering at once), flow-through heating can lag behind a standalone cylinder with a large volume of already-heated water.

For an ordinary family house with standard hot water consumption (2 bathrooms, 4-5 people), a combination of a standalone accumulation tank (800-1,000 l) and a standalone DHW cylinder (150-200 l), connected via a three-way or four-way valve, most often proves itself in practice - it's the solution with the greatest flexibility for servicing, replacing one component without touching the other, and the lowest risk of compromise between the heating regime and the hygiene regime.

Volumes in practice - comparison by household size

Specific recommended volumes differ mainly by heat source (for the accumulation tank) and by number of people/bathrooms (for the DHW cylinder). The following chart shows the commonly recommended ranges we most often see in advisory practice:

The chart shows why mixing up the two devices is so risky: an accumulation tank with a volume of 800-1,000 litres, designed for a solid-fuel boiler, is completely oversized as a DHW cylinder (and additionally doesn't meet the hygiene requirements for material or anti-Legionella protection) - and conversely, a DHW cylinder with a volume of 150-200 litres would be several times undersized as an accumulation tank for a solid-fuel boiler; the boiler would have to be frequently damped down and would burn inefficiently.

Temperature regime - why one can't replace the other

Besides volume, there's also a fundamental difference in how temperature is managed inside the vessel. An accumulation tank is designed so that different temperature layers can exist inside it at the same time (for example 45 °C at the bottom, 75 °C at the top), and the system actively makes use of this difference. A DHW cylinder, on the other hand, needs to be regularly heated throughout its entire volume to a uniform, sufficiently high temperature for hygiene reasons - a partially heated cylinder (for example just the top third) is inadequate protection against Legionella, even if the water "feels" hot when drawn.

This diagram also explains why it makes no sense to set 60 °C as a fixed target for an accumulation tank (it would needlessly waste heat and reduce the efficiency of low-temperature sources like a heat pump), whereas for a DHW cylinder this temperature is a safety necessity that can't be compromised on.

When you need both, and when one device is enough

In practice we come across four typical situations:

The house heats and also needs hot water, and the source is a solid-fuel boiler or a fireplace with an exchanger: here an accumulation tank is practically essential (a wood boiler can't be smoothly regulated like a gas one, it needs "somewhere to put" the surplus heat from intense burning), and the DHW cylinder is either connected to this tank via an exchanger, or is standalone and reheated from it.

The house only has a gas or electric condensing boiler and ordinary radiators: here an accumulation tank is often unnecessary, since a modern condensing boiler can smoothly modulate its output and respond to the current need without a large thermal flywheel. A DHW cylinder (standalone, or combined directly in the boiler as a so-called combi boiler) is still needed here, though, if the house doesn't have flow-through water heating directly in the boiler.

The house has a heat pump: a smaller accumulation tank (200-300 l) is often recommended to hydraulically separate the circuits and reduce the number of compressor starts, extending the device's lifespan. A DHW cylinder here is almost always standalone, since it needs to reach higher temperatures (60-65 °C) than it's energy-efficient for a heat pump to deliver directly into the heating circuit.

The house only has electric water heating (without solid-fuel central heating, or with electric radiator heating): an accumulation tank usually isn't needed at all here, a standalone DHW cylinder (electric or indirectly heated) is enough.

The most common mistakes we see in practice

The first and most common mistake is exactly this mixing up of terms right at the ordering stage - the customer asks the supplier for a "500-litre tank for water" without specifying whether they mean an accumulation tank or a DHW cylinder, the supplier delivers the cheaper variant (usually the accumulation tank, since it doesn't need a hygiene certificate), and it's only discovered when no hot water comes out of the tap, or conversely when water with a metallic taste, or a taste of the heating circuit's water, comes out.

The second common mistake is undersizing the accumulation tank for a solid-fuel boiler - if someone buys, say, only a 300-litre tank instead of the recommended 800-1,000 litres, the boiler has to be damped down during burning, leading to incomplete combustion, tar formation in the chimney, and significantly lower efficiency (up to 15-20% worse fuel utilisation according to boiler manufacturers' typical measurements).

The third mistake is neglecting anti-Legionella protection on the DHW cylinder - especially in households that run the cylinder at a low temperature long-term (for example 45 °C to save energy in combination with a heat pump) without regularly heating it once a week to 60-70 °C. This is a health risk, not just a technical detail, and it doesn't concern the accumulation tank at all (Legionella isn't relevant there, since it's neither drunk from nor used for showering).

A practical example from a typical installation

A typical scenario we come across when advising customers: a family house, 4 people, 2 bathrooms, heating from a combination of a solid-fuel boiler and a gas boiler as a backup source, underfloor heating on the ground floor, radiators upstairs. In this case we recommend an accumulation tank with a volume of roughly 900-1,000 litres (based on the solid-fuel boiler's output, indicatively 50-55 litres per 1 kW of boiler output), with at least four pairs of connections for different temperature levels (a high temperature for radiators, a lower one for underfloor heating, a connection for the DHW exchanger, and a connection for the gas boiler as top-up heating), and a standalone DHW cylinder with a volume of 200 litres, indirectly heated from the accumulation tank via its own exchanger, with an electric heating element as backup for the summer months when the solid-fuel boiler isn't lit.

In a second common scenario - a flat or smaller house with a single gas condensing boiler and no solid fuel - an accumulation tank is usually left out entirely, and either a combi boiler with flow-through DHW heating built into the unit is used, or a standalone, smaller 120-150 litre DHW cylinder connected to the boiler. An investment in an accumulation tank would be wasted here, since a modern condensing boiler can regulate its output smoothly without needing "somewhere to put" surplus heat.

Summary

An accumulation tank and a DHW cylinder solve two different tasks in a heating system - one accumulates heat for heating, the other prepares and hygienically safely stores hot water for personal use. They also work differently in terms of volume and temperature (an accumulation tank at 500-1,500 l across a wide 35-85 °C range, a DHW cylinder at 120-300 l within a narrow, hygienically controlled 55-60 °C range with regular heating up to 60-70 °C). When designing a new system or replacing an old device, it's therefore always important to know exactly which component you need - and if in doubt, it's better to ask in advance than to deal with a shortage of hot water or inefficient heating after installation.

Frequently asked questions

Can an accumulation tank also be used as a DHW cylinder?

No, unless it's specially certified and made of a material suitable for contact with drinking water (for example a stainless-steel or enamelled inner vessel of the tank-in-tank type). An ordinary accumulation tank is made of structural steel intended for a closed heating circuit and doesn't meet the hygiene requirements or the anti-Legionella protection needed for drinking water.

Do I need an accumulation tank if I only have a gas condensing boiler?

In most cases, no. A modern condensing boiler can smoothly modulate its output according to the current need, so it doesn't need a large store for surplus heat. An accumulation tank is recommended more for solid-fuel boilers, combinations of several heat sources, or certain types of heat pumps.

Why does a DHW cylinder have to be regularly heated to a high temperature?

It's a protection against Legionella bacteria, which multiply best in the 25-45 °C range. Regularly (at least once a week) briefly heating the whole volume of the cylinder to 60-70 °C reliably kills off this bacterium. An accumulation tank doesn't address this risk, since no drinking water is drawn from it.

What size DHW cylinder do I need for a family of four?

A volume of 150-200 litres is usually recommended; with more bathrooms, a bathtub, or higher comfort requirements (several people showering at once), a volume of 200-300 litres may be more suitable. The exact calculation also depends on the exchanger's output, i.e. how fast the cylinder can reheat after a large draw.

Is a combined tank (tank-in-tank) better than two separate vessels?

It's not clear-cut - a combined tank saves space and usually heats DHW faster thanks to its large heat exchange area, but is more expensive and, if the inner vessel fails, servicing is more complicated (often the whole device has to be replaced). Two separate vessels are cheaper to buy and to service, but need more space and a more complex hydraulic connection.

Can I have just an accumulation tank at home without a DHW cylinder?

Yes, as long as you handle hot water another way - for example flow-through heating directly in a combi boiler, a standalone electric heater, or if the house doesn't need hot water from a central system at all. An accumulation tank on its own only addresses the heating circuit, not preparing hot water.

Do you have a question on this topic?

Can't decide whether your system needs an accumulation tank, a DHW cylinder, or both? Write to us - we're happy to help.